In a typical ion implanter, a relatively small cross-section beam of dopant ions is scanned relative to a silicon wafer or other substrate. This can be done in one of three ways: scanning of the beam in two directions relative to a stationary wafer, scanning of the wafer in two directions relative to a stationary beam, or a hybrid technique in which the beam is scanned in one direction while the wafer is mechanically scanned in a second, typically orthogonal direction.
Batch implanters, designed for the simultaneous implantation of a batch of wafers in the end station of an implanter, typically have a rotating wheel carrying a number of wafers around its periphery. The wheel is rotated so that each wafer passes through the ion beam in turn and then the axis of rotation of the wheel is reciprocated to translate the circumferential path of the wafers on the wheel substantially radially across the beam to ensure all parts of the wafer are evenly implanted. An example of such a scanning wheel type batch implanter is disclosed in U.S. Pat. No. 5,389,793.
A system for scanning the ion beam in two directions relative to a stationary wafer is disclosed in U.S. Pat. No. 4,736,107 and hybrid techniques with the beam scanned in one direction and the wafer mechanically scanned in an orthogonal direction are disclosed in U.S. Pat. No. 5,898,179, U.S. Pat. No. 5,003,183, U.S. Pat. No. 5,229,615 and U.S. Pat. No. 5,406,088.
Two dimensional beam scanning systems and hybrid scanning systems are normally favoured for implanting single wafers one at a time. However, arrangements involving scanning the ion beam present certain challenges to the design of successful implanters. Scanning the ion beam with varying magnetic fields is favoured over electrostatic beam scanning, because the presence of electric fields removes neutralising electrons from the beam plasma, causing space charge resulting in beam control problems. This is especially severe at low energies and high beam currents. However, beam scanning systems, especially magnetic scanning systems, are bulky and relatively expensive, and require an extended beam line, which can substantially increase the overall footprint of the ion implanter in the semiconductor fabrication facility.